When serializing a XDocument with "xmlns" elements, they usually come
with a prefix like in xmlns:xlink="http://www.w3.org/1999/xlink".

When the prefix doesn exist like in xmlns="", the prefix member of the
xmlNsPtr structure is NULL and unsafe operations on it like "strlen"
may cause a SIGSEV.

Change-Id: Icdfcddeccb37c246a48aa1239b7f3c5b0b3727e3
Reviewed-on: https://gerrit.libreoffice.org/6003Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>